Various chemicals are being administered to various species of Old World monkeys to obtain comparative data on the response of primates and rodents to known carcinogens and to materials suspected to be carcinogenic for humans. In addition, it is hoped that biological markers which will aid in the diagnosis of preneoplastic change as well as for frank neoplasia will be developed. Thus far seven chemicals have induced tumors in Old World monkeys; these are diethylnitrosamine, dipropylnitrosamine, 1-nitrosopiperidine, methylnitrosourea, methylazoxymethanol acetate, aflatoxin B1 and procarbazine. Methylnitrosourea has induced squamous cell carcinomas of the oral cavity; procarbazine has induced acute myelogenous leukemia, lymphoma and hemangiosarcoma while the other carcinogens have all induced primary liver carcinomas. The serum levels of alpha-fetoprotein by radioimmunoassay in monkeys with biopsy-proven liver tumor were scattered over a wide range of 100,000 fold variation which is analogous to the range seen in human hepatocellular carcinoma. Thus far surgery is the only modality that has consistently reduced the mass of primary liver tumor more than 50%, although several chemotherapeutic agents have reduced the tumor mass of the alpha-fetoprotein level by more than 50% in certain monkeys.